Follow up with the IRS again. Also contact the IRS Taxpayer Advocate to see if they can help since it sounds, from your description, as if the IRS is not properly following through on its duties. Also, consider filing a complaint with the local police for identity theft. If the person whom you suspect is claiming the deduction is not a relative of your son and has no legitimate reason for having your son's social security number, then that person might have engaged in some form of identity theft.
My answer does not constitute legal advice and may not be relied upon by anyone for any purpose and does not constitute an attorney/client relationship or an offer to form such a relationship. This disclaimer is intended to be fully compliant with the requirements of Treasury Department Circular 230 and the terms thereof are fully incorporated by reference. If you wish to consult with me please contact me at dana@nytaxcounsel or visit my website at www.nytaxcounsel.com
As Dana suggested, follow up with the IRS. It is imperative you document your correspondence with them - write down date/time/length of call/information discussed and definitely keep the badge number of the person you speak with.
You are doing everything right it sounds like - and dealing with a large bureaucracy is often difficult and can be frustrating at times - but the most important thing when you are doing the right thing, is to document your efforts.
Do not threaten the "wrongdoers" in any kind of fashion, no matter how benign of a threat you think it is, because then you can find yourself in serious legal trouble - especially if you do not have convincing evidence (and it sounds like at the moment, you do not have the 'smoking gun.')
Try to sort it out with the IRS first, and there are other options later if it comes to that. However with your documentation, even if you do hit audit, it shows a shortcoming on the part of the IRS, not your own.
Best of luck
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